Brushless DC motors are finding increased use in specialized fields such as servo motors, magnetic tape drives, magnetic disk drives, and other applications in which it is desirable to control with precision the motor current and torque. At times, it is desirable that a large current be applied to a motor to rapidly accelerate it to its full speed. In other instances, it is desirable that the motor be stopped by applying a braking current to the motor. Precision control of the stopping and starting currents is required in order to achieve these objectives, while at the same time, preventing the currents from exceeding levels that could damage the motor or control circuit elements such as power transistors and other solid state devices.
It is known to use specialized circuitry for controlling motor currents. The book entitled, "Brushless DC Motors Electronic Commutation and Controls," by Thomas J. Sokira and Wolfgang Jaffe, 1st Ed., 1990, published by Tab Books, Inc. shows in FIG. 6-1 on page 170 the elements of a current control system for a three-phase brushless DC motor. FIG. 6-29 on page 210 shows further details of motor current monitoring systems. This includes a sampling resistor connected in a path common to all phases of the motor to monitor the total motor current. While the circuitry of FIG. 6-29 does monitor the total motor current, it is less than optimum since it is sometimes desirable to monitor the currents in each separate phase of the motor. FIG. 6-28 of the above publication on page 209 shows circuitry which permits the current in each phase to be individually monitored. This is done by using a low valued sensing resistor in each leg and by monitoring the voltage drop across each sensing resistor to determine the current in each leg. While the circuitry of FIG. 6-28 is advantageous over that of FIG. 6-29 in that it permits the phase currents to be individually monitored, the circuitry of FIG. 6- 28 still is less than ideal since it only permits unidirectional currents to be monitored. This is a significant limitation since reverse currents are sometimes encountered during the acceleration, running, and deceleration of a motor. This limitation precludes the optimum control of the current and motor torque required in certain applications for such motors.
It can therefore be seen from the above that it is a problem to accurately monitor currents of all types, including bi-directional currents in the separate phases of multi-phase brushless DC motors. This capability is necessary for one to be able to monitor and control with precision the torque the motor develops and to avoid damaging the motor or the drive circuitry.